The invention relates to a process as claimed in the preamble of claim 1 and a device as claimed in the preamble of claim 6.
The intensity distribution of a material machining laser beam on a tool surface has for a long time been adjusted with focussing lenses of varied focal distance. The-imaging of laser beams can be described with Gaussian beam theory. In simplified terms it can be stated here: The wider a collimated laser beam striking a focussing lens, the smaller its focal diameter and the shorter its focussing depth. The intensity distribution on a tool surface to be machined was thus adjusted in the past by a suitable choice of a focussing lens with a corresponding preceding beam widening.
By a suitable choice of so-called mode diaphragms at the xe2x80x9ccorrectxe2x80x9d location in the laser resonator moreover the generated laser mode was changed. A mode diaphragm with a small cross section yielded a fundamental mode while mode diaphragms with a large opening yielded a wide multimode beam.
When high powers were used, laser beams of several lasers were combined coaxially on top of one another. Thus the intensity distribution on a tool surface to be machined could be changed only by changes in the optical structure. If a high speed intensity change was to be carried out, using optical switching elements such as Pockels cells (electro-optical effect), Faraday rotators (magneto-optical effect) or by acousto-optical modulators only the entire radiated-in power could be changed. In doing so however only attenuation over the entire beam cross section took place. The relative intensity distribution over the focussed beam section was thus preserved, aside from the diffraction effects and spatial distortions. The intensity distribution consisted of a numerical factor and a local function [l=axc3x97f(x,y)], and only the numerical factor [a] could be changed in the known processes.
The object of the invention is to devise a process and a device with which the curve shape of a an intensity distribution of a laser beam at the machining site can be quickly changed, i.e. with switching times down to submicroseconds. A change of the curve shape of the intensity distribution cannot be done simply by attenuation of the entire material machining beam; this would yield the same curve shape again except for diffraction effects and distortions. The object of the invention is to change no longer only the numerical factors in the intensity distribution, but also the value of the function:
xe2x80x83I1≈axc3x97f1(x, y, t); I2=axc3x97f2(x, y, t)
Of course the linear factor a can also be varied.
The object is achieved by at least two component beams with different intensity distribution being used and being combined into one working beam. This working beam is then directed preferably focussed on the surface of an object for machining. In contrast to conventional methods, at this point the first intensity distribution Ib=axc3x97fb(x, y, t) differs from the following one In=axc3x97fn(x, y, t) by the value of the function fu(x, y, t) and no longer only by an altered linear value a. Here x and y are the coordinates of a plane, preferably over the beam cross section, and t is a time function.
Because the intensity of the component beams is changed individually, when the beams are combined no longer does a linear change of the curve shape of the intensity distribution take place, but the curve shape changes as a whole. A change of the curve shape, as indicated below, yields different drilling behavior depending on whether the intensity of the center of the machining beam or on the edge of the beam is increased.
As claimed in the invention at least two component beams I1≈a1xc3x97f1(x, y, t) and I2=a2(x, y, t) are combined, and the factors a1 and a2 can be changed with switching times (change times) down to submicroseconds. The combination of the two beams xcexa3 I2+I2 then yields a working beam with a different function f3 (x, y, t) of the intensity distribution over the beam cross section. In this way the diameter of the working beams can be xe2x80x9cpromptlyxe2x80x9d controlled. Thus, on a workpiece to be machined not only does an adjustable intensity profile arise, but also an adjustable or definable depth sharpness profile.
The invention can be used to advantage wherever material must be removed with a depth and width which change rapidly in space. One preferred application is the production of screen cells on an engraved cylinder. Screen cell production using laser radiation is known from WO 96/34718. But in the known process an intensity distribution was used as can be achieved by focussing a single beam. The advantageous intensity distribution which can be adjusted almost at will, as achieved by the invention by combining several beams, was unknown to WO 96/34718.
With the device described below the component beams can be turned on and off or partially turned on and off at different times. That is, not only is a rapid variation of the intensity of the intensity distribution over the beam cross section possible, but also a time variation: xcexa3 I2+I2+. . . =xcexa3 fa1 (t)xc3x97f1 (x, y, t)+fa2 (t)xc3x97f2 (x, y, t). This type of variation will be used wherever brief heat-up effects (for example, formation of a plasma cloud) are important.
Material removal is not essential for the use of the invention: use is also possible in laser inscribing without the removal of material and also in material hardening. In addition, objects can also be illuminated (for example modulated pumping of laser crystals or media). Pulsed or also continuously operating laser beams can be used.